This invention relates to reducing phase error in received composite low frequency information on a high frequency carrier, such as transmitted by an FMX broadcast system.
The FMX system transmits the three standard components of a conventional FM stereo broadcast. These components are: (1) the M signal, which is the monocompatible baseband audio signal, comprised of the sum of the left and right audio signals (L+R), frequency modulated onto the high frequency carrier; (2) the S signal, which is the difference between the left and right audio signals (L-R) modulated onto a 38 kHz subcarrier using double-sideband suppressed carrier (DSB-SC) modulation; and (3) the P signal which is a pilot 19 kHz subcarrier used by the receiver to regenerate a 38 kHz sub-carrier or reference signal for demodulating the S signal. The FMX system adds a fourth signal called the S' signal, which is also derived from the difference between the left and right audio signals (L-R), but is filtered, compressed, and DSB-SC modulated onto the 38 kHz subcarrier in phase quadrature with the S signal. The S' signal is normally not detected by a conventional FM stereo receiver. An FMX receiver, however, uses the S' signal instead of the S signal. The S' signal is marked by a 10 Hz identification tone. FMX receivers may use the S signal to derive the compression algorithm used to compress the dynamic range of the S' signal, which allows for adaptive expansion of the dynamic range in the receiver. U.S. Pat. No. 4,485,483 entitled "FM Stereophonic System Incorporating Companding of Difference Signal" of Torick et al. and the articles entitled "Improving the Signal-to-Noise Ratio and Coverage of FM Stereophonic Broadcasts" in 33 J. Audio. Eng. Soc. 938 (Dec. 1985) and "Reentrant Compression and Adaptive Expansion for Optimized Noise Reduction," 33 J. Audio Eng. Soc. 944 (Dec. 1985) describe the FMX broadcast service.